1. Field of the Invention
The invention is directed to a continuous scrap feed into an electric smelting furnace (EAF), a channel that is connected at one end to an opening in the wall of the smelting furnace and has at its other end a charging device for the scrap, and an extraction device for the off-gases from the smelting process which are guided through the channel and which serve to preheat the scrap, this extraction device being connected to the channel between the two ends of the channel, wherein the extraction device is located closer to the end of the channel facing the smelting furnace than to the end facing the scrap feed in relation to the total length of the channel.
2. Related Art
Continuous scrap feed into an electric arc furnace is known, for example, from EP 744 585 B1, EP 190 313 B2, U.S. Pat. No. 5,400,358, or WO 20071006558 A2.
With an edge length of less than 1.5 m, a size of the scrap pieces on a feed belt can be very large. It can be deduced from this that a corresponding opening in the side of the EAF will be about 3 to 4 m2. At all events, this opening would have to be closed during smelting with conventional dust extraction technology because the infiltration of false air into the smelting vessel would otherwise be too high.
At present, a blower for extracting process gases in continuous scrap feeding is typically located in the area of the scrap feed so that the EAF gases are drawn a long distance over the charged scrap.
Two past suggestions for reducing the infiltration of false air into the EAF and increasing the efficiency of dust extraction were:
1. Positioning a second blower next to the exhaust and controlling its output by negative pressure in the scrap supply tunnel of the EAF; or
2. Providing a mechanical barrier to increase air resistance at the inlet into the scrap feed.
Control for purposes of the first solution is very costly and cannot be monitored satisfactorily. Further, it is very maintenance-intensive. An additional amount of electrical energy is consumed by the second blower.
The second solution employs flow resistances in the scrap supply tunnel that must be precisely scaled to suction power and the amount of process gases.
In both solutions, the furnace gas (depending on construction and furnace size) is extracted along a length of 30 to 50 m so that it is cooled and must be heated again to a temperature of >700° C. (temperature is a function of the O2 content in the off-gas) by an auxiliary burner to suppress emission of furans and dioxins.
It has already been proposed that the extraction device be positioned closer to the end of the channel facing the smelting furnace than to the end facing the scrap feed in relation to the total length of the channel.
Scrap and off-gases move in opposite directions in the channel; while the scrap is conveyed into the smelting furnace, the off-gas is drawn away from the smelting furnace.
The extraction device and its shaft for extracting the off-gases from the EAF, which is constructed with a continuous charging, are positioned at the channel in such a way that the extraction device and its shaft are in proximity to the smelting furnace and the distance from the position where the scrap and (possible) additions are fed is appreciably greater. In addition, movable flaps can be installed to increase the flow resistance in the area of the scrap feed and, therefore, to more fully exploit the capacity of the extraction device in relation to the arc furnace.
The ratio of channel length to the EAF to channel length to the scrap feed could be 1 to 2, for example.
Analyses of existing installed EAFs with continuous scrap charging has shown that the heat of an appreciable subsequent combustion of high-CO off-gases that can be delivered to the supplied scrap is locally limited to only the first few meters behind the smelting furnace. Also, the sensible heat of the off-gases can only be made use of to a limited degree in this area.
Because of the residual heat in the off-gas, a positioning of the intake connection as a function of the amount of off-gas and the output of the EAF requires, if any, only a limited additional amount of energy needed to adjust the off-gas temperature and, accordingly, to suppress furans and dioxins.